Electrolysis is a common form of electrochemical refining. In an electrolysis process, the ore is dissolved in an aqueous or non-aqueous solution or melted in an electrolytic furnace. Once dissolved or melted, the ore dissociates into ionic species, forming an electrolyte. The metallic components of the ore to be extracted become positively charged cations. The remaining components, typically oxygen and halogens, become negatively charged anions. To extract the metal from the ore, an electric potential is applied across two electrodes that are immersed in the electrolyte. The metallic ions are thereby attracted to the negatively charged cathode, where they combine with electrons and are deposited as metal. The counter ions, most typically oxygen and halogens, are driven to the positively charged anode and evolve as waste gases.
Oxygen-ion-conducting membranes, such as yttria-stabilized zirconia, have been used in electrolytic metal extraction processes and permit the extraction of pure metals from their respective oxides. A metal oxide dissolved in a suitable molten electrolyte is separated from the anode by oxygen-ion-conducting membrane. When a potential is generated between the cathode (in contact with the electrolyte) and the anode (in contact with the oxygen-ion-conducting membrane), and the dissociation potential of the oxides are exceeded, the oxygen species of the electrolyte is transported across the ion-conducting membrane and is oxidized at the anode, and the metallic species are reduced at the cathode. This electrochemical cell is referred to as a solid oxide membrane (SOM) cell.
The oxidizing environment of the anode limits the available choices for an anode. In order to reduce the oxidizing environment of the anode, a consumable carbon source or consumable reducing gas such as carbon monoxide (CO) and/or hydrogen is typically continuously fed over the anode to getter or react with the oxygen generated at the anode. This reduces the corrosive oxidative environment at the anode; however, reducing gases and in particular hydrogen are expensive and can add significantly to the cost of metal extraction. Due to the amount of reductant gas required, H2 gas has to be fed in continuously. This puts technique challenges on hydrogen transportation, storage and safety.
An electrolysis cell useful in the synthesis of metals from their respective oxides is desired; in particular, anodes that are stable under oxidizing conditions of the anode are desired. An electrolysis system that does not require a reductant is desired.